(a) Field of the Invention
The present invention relates to a display device and a driving method thereof, and more particularly to an organic electroluminescent (EL) light emitting display utilizing EL light emission of an organic material, and a method for driving the organic EL light emitting display.
(b) Description of the Related Art
Generally, organic EL displays are display devices that emit light by electrically exciting an organic compound. Such an organic EL display includes n×m organic light emitting cells arranged in the form of a matrix, and displays an image by driving the organic light emitting cells, using voltage or current.
Organic light emitting cells can also be referred to as “organic light emitting diodes (OLEDs)” because they have diode characteristics. As shown in FIG. 1, each organic light emitting cell has a structure including an anode electrode, an organic thin film, and a cathode electrode. The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) to improve balance between electrons and holes and to improve light emitting efficiency. The organic thin film also includes an electron injecting layer (EIL) and a hole injecting layer (HIL). As discussed above, organic light emitting cells are arranged in an n×m matrix to form an organic EL display panel of an organic EL display. In addition, when transparent electrodes are used for both the anode and cathode electrodes of an organic light emitting cell, it is possible to implement a double-sided organic EL display.
Driving methods for an organic EL display panel can be classified as either a passive matrix type driving method or an active matrix type driving method using thin film transistors (TFTs). In accordance with the passive matrix type driving method, anodes and cathodes are arranged to be orthogonal to each other so that a desired line to be driven is selected. In accordance with the active matrix type driving method, thin film transistors are coupled to respective indium tin oxide (ITO) pixel electrodes in an organic EL display panel so that the organic EL display panel is driven by a voltage maintained by the capacitance of a capacitor coupled to the gate of each thin film transistor.
FIG. 2 is a block diagram schematically illustrating an organic EL display including the organic EL element.
As shown in FIG. 2, the organic EL display includes an organic EL display panel 100, a scan driver 200, and a data driver 300.
The organic EL display panel 100 includes a plurality of data lines D1 to Dm extending in a column direction, a plurality of scan lines S1 to Sn extending in a row direction, and a plurality of pixel circuits 110. Each of the data lines D1 to Dm transmits a data signal indicative of an image signal to respective ones of the pixel circuits 110. Each of the scan lines S1 to Sn transmits a scan signal to respective ones of the pixel circuits 110. Each pixel circuit 110 is formed at a pixel region defined by neighboring ones of the data lines D1 to Dm and neighboring ones of the scan lines S1 to Sn. Hereinafter, pixel circuits (or pixels) corresponding to the pixel circuits 110 are denoted in association with scan lines, to which the pixel circuits are coupled. For example, the pixel circuits (or pixels) coupled to the scan line S1 are denoted by “P1”, and the pixel circuits (or pixels) coupled to the scan line Sn are denoted by “Pn”.
The scan driver 200 applies a scan signal to the scan lines S1 to Sn in a sequential manner. The data driver 300 then applies data voltages corresponding to input image signals to the data lines D1 to Dm, respectively.
The scan driver 200 and/or data driver 300 may be coupled to the display panel 100. Alternatively, the scan driver 200 and/or data driver 300 may be mounted, in a chip, on a flexible printed circuit (FPC) or a film bonded to the display panel 100 and coupled to the display panel 100. Alternatively, the scan driver 200 and/or data driver 300 may be directly mounted on a glass substrate of the display panel 100. Also, the scan driver 200 and/or data driver 300 may be directly mounted on the glass substrate so that the scan driver 200 and/or data driver 300 may be substituted for drive circuits respectively formed on the same layers as those of the scan lines, data lines, and thin film transistors.
Korean Patent Laid-open Publication No. 2002-0097420 discloses a bi-directional data driver including a bi-directional shift register to bi-directionally apply a data signal, the entire content of which is incorporated herein by reference. That is, in an organic EL display capable of implementing double-sided display, images displayed on the front and back screens of the organic EL display are horizontally inverted from each other (e.g., left to right and right to left). In order to display the same image on the front and back screens, accordingly, the order of applying data signals to the data lines in association with the image display on the front screen must be bi-directionally applied or reverse to the order of applying the data signals to the data lines in association with the image display on the back screen. For example, the m-th (or last) data signal to be applied to the data line Dm for the image display on the front screen must be applied to the data line D1 for the image display on the back screen.
On the other hand, where it is desired to display the same image even when the display panel is inverted in the vertical direction as well as in the horizontal direction, for example, in accordance with a 180° rotation thereof (e.g., up to down and down to up or top to bottom and bottom to top), the scan driver must also use a bidirectional shift register to bi-directionally apply a scan signal, similar to the application of the data signal by the bi-directional data driver. That is, in the case of an EL display including a 180°-rotatable display panel, a bi-directional scan driver is used to change the order of sequentially applying scan signals to scan lines between the sequential selection of the scan lines in a downward direction (hereinafter, referred to as “forward scan”) and the sequential selection of the scan lines in an upward direction (hereinafter, referred to as “backward scan”), and thus, to display the same image on the screen in both the non-rotated state and the rotated state. For example, the bi-directional scan driver applies the first scan signal, to be the scan line S1 in a forward scan mode, to the scan line Sn in a backward scan mode, and applies the n-th scan signal, to be applied to the scan line Sn in the forward scan mode, to the scan line S1 in the backward scan mode.
In accordance with the above-mentioned conventional techniques, however, there is a problem in driving certain pixel circuits. For example, in a pixel circuit configuration disclosed in Korean Patent Laid-open Publication No. 2004-0009285, the entire content of which is incorporated herein by reference, each pixel circuit can operate, based on at least two different scan signals. For example, the pixel circuit Pn can operate, based on the n-th scan signal applied to the current scan line Sn and the “n−1”-th scan signal applied to the preceding scan line Sn−1. In particular, the pixel circuit Pn is arranged to operate normally in the forward scan mode in accordance with the “n−1”-th scan signal applied to the scan line Sn−1 and the n-th scan signal subsequently applied to the scan line Sn. However, this pixel circuit Pn cannot properly (or normally) operate in the backward scan mode when the application order of scan signals to the scan lines is reversed such that the first (or previous) scan signal is applied to the scan line Sn (or current scan line), and the second (or next or current) scan signal is then applied to the scan line Sn−1 (or previous scan line).